Enhancing Defect-Induced Dipole Polarization Strategy of SiC@MoO3 Nanocomposite Towards Electromagnetic Wave Absorption

Highlights Oxygen-vacancy-rich SiC@MoO3 nanocomposite with strong reflection loss (− 50.49 dB at 1.27 mm thickness) and broadband absorption band (8.72 GHz at 2.68 mm thickness) were constructed via in situ etching strategy. The presence of oxygen vacancy leads to an increased conductive loss and defect-induced dipole polarization, which plays significant role in attenuating the incident electromagnetic wave. Supplementary Information The online version contains supplementary material available at 10.1007/s40820-024-01478-2.


Introduction
With the advancement of technological innovation, wireless communication facilitated by electromagnetic wave has significantly reduced the spatial distances among individuals.Simultaneously, the widespread integration of smart appliances and advanced office automation equipment is progressively reshaping societal habits.However, the ongoing degradation of the electromagnetic environment not only disrupts the reliable operation of precision instruments but also risks causing irreversible harm to human health [1][2][3].Consequently, scholars universally agree on the urgent need to develop innovative electromagnetic wave-absorbing materials that are highly performant, easily manufacturable, and environmentally sustainable, with the goal of effectively mitigating the harmful impact of electromagnetic waves [4,5].To date, a diverse array of electromagnetic wave-absorbing materials has emerged, encompassing carbon-based substances, ceramics, metal composites, conductive polymers, and meta-materials [6][7][8].
Transition metal oxides (TMOs) are recognized as conventional dielectric loss absorbers that can effectively attenuate incident electromagnetic waves over a broad frequency range through conductive loss and dielectric polarization, thereby achieving better performances than magnetic loss absorbers [9,10].Among them, molybdenum oxide (MoO 3 ), noted for its distinctive layered structure, low density, wide availability of raw materials, ease of preparation, corrosion resistance, and environmental compatibility, has emerged as a prominent member of the dielectric loss absorber family.However, bare MoO 3 often exhibits suboptimal electrical conductivity, limited relaxation polarization channels, and poor impedance matching.These shortcomings lead to weak reflection loss, a narrow effective absorption bandwidth, and undesirable matching thickness.These limitations have persistently resulted in inferior electromagnetic wave absorption performance, impeding its broader application [11][12][13].
For dielectric loss absorbers, interfacial polarization, dipole polarization, and relaxation polarization play significant roles in the attenuation of electromagnetic waves.Research has revealed that employing materials with high electrical conductivity as substrates/carrier and combining them with MoO 3 to fabricate MoO 3 -based nanocomposite absorbers enhances not only their electrical properties but also leverages the interfacial polarization at the composite interface [14][15][16].This methodology significantly enhances the complex permittivity of the absorber, thereby significantly increasing the attenuation of electromagnetic waves and consequently yielding an absorption performance superior to that of single-component absorbers.
Building upon the previously outlined research strategy, researchers have already attained some valuable results in the study of the electromagnetic wave absorption performance of MoO 3 -based nanocomposite.However, several critical challenges persist, mainly including (1) The proportion of the biphase interface area on the surfaces of MoO 3 and the substrate components is exceedingly small, resulting in a limited interfacial polarization effect.This fails to fully exploit the advantage of the high specific surface area of nanomaterials.(2) The strategies adopted to enhance electromagnetic wave absorption performance are consistent with those used for non-semiconducting transition metal oxides, neglecting the semiconducting nature of MoO 3 and not intervening in the electron transport process.(3) The attenuation effect of oxygen vacancies on electromagnetic wave is commonly ascribed to induced dipole polarization, yet there is a lack of microscopic understanding and investigation of the fundamental mechanisms underlying electromagnetic attenuation.
Currently, the induction of oxygen vacancies has become one of the prevalent methods for modulating the physical properties of semiconductor nanomaterials [17,18].Research has shown that the introduction of oxygen vacancies can create defect states within the band gap, which alters the band edges and band gap structure, providing additional free electrons that increase electrical conductivity.Additionally, this can affect the magnetic ordering and exchange interactions of intrinsic products, and also enhance photocatalytic activity by trapping charge carriers and reducing recombination rates [19][20][21][22].This consequently improves impedance matching and increases the dielectric loss of the materials in an alternating electromagnetic field, thereby attaining electromagnetic wave absorption performance that surpasses pristine samples.
In this study, MoO 3 , characterized as a wide-band gap semiconductor, served as the subject of research, beginning with the modulation of its electronic structure.Using SiC nanowires as a carrier, SiC@MoO 3 nanocomposites were successfully synthesized via electro-deposition and calcination processes.Subsequently, through in-situ etching with KBH 4 , SiC@MO-t samples exhibiting diverse morphology and oxygen vacancy concentrations were prepared.By introducing oxygen vacancy defects into the intrinsic products, the conductive and induced dipole polarization loss effects associated with MoO 3 upon incident electromagnetic waves were enhanced.This simultaneously improved the impedance matching of these products, resulting in improved electromagnetic wave absorption performance.Furthermore, in concert with insights from density functional theory calculations, a mechanism understanding of how oxygen vacancies contribute to the enhanced electromagnetic wave absorption performance of SiC@MoO 3 nanocomposites has been proposed.This study provides a systematic framework for employing vacancy engineering to enhance the electromagnetic wave absorption properties of TMOs and paves the way for the development of efficient, scalable absorbing materials.

Preparation of SiC@MoO 3 Nanocomposite
In this work, the substrate material was SiC nanowires, a product of our past research [23].Initially, the SiC nanowires were immersed in a 3 M sodium hydroxide solution to eliminate the impurities on the surface.Subsequently, the residual impurities were removed by rinsing with deionized water followed by immersion to achieve neutrality for later use.Electro-deposition was carried out in a standard three-electrode configuration, with SiC nanowires serving as the working electrode, Pt wires and a saturated calomel electrode (SCE) were employed as the counter electrode and reference electrode, respectively.MoO 3 deposition on SiC nanowires proceeded from an aqueous electrolyte solution of 15 mmol L −1 (NH 4 ) 6 Mo 7 O 24 •4H 2 O using a CHI660e (Chenhua Instrument Co., Shanghai, China) electrochemical workstation operating at − 10 mA cm −2 for 400 s.The SiC@ MoO 3 nanocomposite was then obtained through calcination at 350 °C for 90 min under air at a ramp rate of 2 °C min −1 .

Preparation of Various SiC@MO-t Samples
The as-prepared SiC@MoO 3 nanocomposites were immersed in a 2 M KBH 4 aqueous solution for 2, 4, 6, and 8 min, respectively, resulting in the acquisition of nanocomposites with different oxygen vacancy concentrations.These products were denoted as SiC@MO-t2, SiC@MO-t4, SiC@ MO-t6, and SiC@MO-t8, respectively.Figure 1a depicts the preparation processes of SiC@MoO 3 nanocomposites and various SiC@MO-t samples, where the red dotted rings in the ball-and-stick model indicate different concentrations of oxygen vacancies.

Characterization
The morphology of the nanocomposites was examined using a scanning electron microscope (SEM, Hitachi S-4800), while the micro-structure was verified using transmission electron microscopy (TEM, Hitachi H-8100).The phase composition of the SiC@MoO 3 nanocomposite and SiC@ MO-t samples was examined using a Rigaku D/max-2400 powder X-ray diffractometer (XRD).The chemical bonding and surface valence states were analyzed using X-ray Photoelectron Spectroscopy (XPS, Thermo ESCALAB 250XI).Electron paramagnetic resonance (EPR) measurements were performed using a Germany Bruker EMX plus paramagnetic resonance spectrometer.The resistance was measured using a multifunctional digital four-probe tester (ST-2258C multifunction digital four-probe tester).To evaluate the electromagnetic wave absorption properties, the samples with a filling ratio of ~ 30 wt% were combined with paraffin wax and compressed into a small ring with an inner diameter of 3 mm, an outer diameter of 7 mm, and a thickness of 3.04 mm.The electromagnetic parameters were measured in the 2-18 GHz frequency range using a Vector Network Analyzer (Agilent, N5230A).[24].Moreover, different SiC nanowires within the field of view intertwine randomly, forming a dense network structure.According to previous studies, the presence of this unique intertwined structure may enhance electron mobility and lead to significant conductive loss for incident electromagnetic waves [25][26][27].Figure 1b-d presents representative SEM images of the SiC@MoO 3 nanocomposite.The surface of the bare SiC nanowires is covered by flaky MoO 3 , with adjacent flakes overlapping at various angles, forming a thick shell.As depicted in Fig. 1d, the MoO 3 flakes consist of numerous, orderly stacked nanosheets.Additionally, after successful modification with MoO 3 flakes, the diameter of the nanowires increased to 1.1 μm, and their surfaces became significantly rougher, enhancing the potential for multiple reflection and scattering effects compared to those of smooth pure SiC nanowires [28].Figure 1e-i illustrates the elemental mapping of the SiC@ MoO 3 nanocomposite.The elements C and Si are predominantly located in the central region, corresponding to the size and position of the SiC nanowire core.The elements Mo and O are primarily distributed on either side of the Si and C elements and originate predominantly from the MoO 3 nanoshell.Figure S2 presents a representative EDS spectrum of the SiC@MoO 3 nanocomposite.The components and their relative amounts identified in the spectrum closely correspond to those observed in the elemental mapping.Figure 1k presents the TEM image of a single SiC@ MoO 3 nanocomposite with a diameter of approximately 1 μm, a dimension consistent with the results shown in the SEM images of Fig. 1b.To further refine the lattice structure information of the product, HRTEM analyses were conducted on both the SiC nanowire core and the external MoO 3 shell, with the results presented in Fig. 1l-n.Figure 1l illustrates distinct lattice fringes with a spacing of approximately 0.25 nm between adjacent crystal planes.This spacing is consistent with the cubic SiC (111) plane (JCPDS No. , indicating that the SiC core is preferentially oriented along the [111] direction [29].Additionally, some dark lattice fringes within the field of view have been noted, which can be attributed to stacking faults (S.F.) in the lattice according to previous studies [30].Figure 1m, n presents the HRTEM images of the MoO 3 shell, taken from the yellow and green square areas in Fig. 1k, respectively.The images show interplanar spacing of 0.35 and 0.32 nm, corresponding to the (400) and (210) planes of MoO 3 [31].These spacing indicate that the MoO 3 grains exhibit varied crystal orientations across different thin film locations, demonstrating an overall poly-crystalline structure.
Figure 2a-d depicts the SEM, TEM, HRTEM, and EDS mapping images of SiC@MoO 3 nanocomposites fabricated under various KBH 4 soaking time conditions.SEM characterization results demonstrate that after soaking treatment, various SiC@MO-t samples preserve their original linear morphology.However, compared to untreated samples, the surface of MoO 3 nanosheets grows increasingly rougher, and the diameter of the nanowires progressively decrease as soaking time is extended.TEM images of samples prepared under various soaking times reveal that, due to the potent reducing action of KBH 4 , MoO 3 nanosheet structures, initially hundreds of nanometers thick, are reduced to smaller, tens of nanometers thick.
The surface contour undergoes a gradual transition from 150 degrees (SiC@MO-t2, as depicted in Fig. 2a2) to 120 degrees (SiC@MO-t4, as illustrated in Fig. 2b2), and then to 100 degrees (SiC@MO-t6, as depicted in Fig. 2c2), ultimately leading to a coral-like morphology (SiC@MO-t8, as illustrated in Fig. 2d2), the schematic illustration was shown in Fig. 2e. Figure 2a3 presents a typical HRTEM image of the SiC@MO-t2 sample, displaying clear lattice fringes.The spacing between adjacent crystal planes is approximately 0.32 nm, corresponding to the (210) crystal plane of MoO 3 [32].At the edge of the crystalline area, very small dark spots are scattered (highlighted by yellow circles), likely resulting from the reduction of oxygen atoms in the MoO 3 lattice by KBH 4 , resulting in oxygen vacancies.In Figs.2b3-d3, besides the corresponding interplanar spacings that can be respectively labeled as the (201), (101), and (210) planes, oxygen vacancy defects similar to those in the SiC@MO-t2 sample can also be observed within the field of view.Additionally, as the soaking time increases, the generated oxygen vacancies gradually extend from the crystal Fig. 2 a1-a7 SEM, TEM, HRTEM images and EDS mapping elements dispersion of SiC@MO-t2 nanocomposite.b1-b7 SEM, TEM, HRTEM images and EDS mapping elements dispersion of SiC@MO-t4 nanocomposite.c1-c7 SEM, TEM, HRTEM images and EDS mapping elements dispersion of SiC@MO-t6 nanocomposite.d1-d7 SEM, TEM, HRTEM images and EDS mapping elements dispersion of SiC@MO-t8 nanocomposite.e Schematic illustration of the etching process of SiC@MO-t samples surface to the crystal interior (as indicated by the pink arrows in the figures).Notably, in Fig. 2c3, d3, small amorphous regions appear within the MoO 3 lattice (highlighted by blue curves), possibly due to the disordered arrangement of atomic structures resulting from a high concentration of oxygen vacancies [20,33].The O and Mo elemental distribution mappings of various samples exhibit phenomena consistent with the described changes in nanowire diameter and morphology.
Figure 3a displays the XRD patterns of the SiC@MoO 3 nanocomposite and SiC@MO-t samples.Compared with the XRD pattern of bare SiC nanowires, the major diffraction peaks at 35.6°, 41.4°, 59.9°, 71.7°, and 75.5° are identified as the cubic SiC (JCPDS card No. 29-1129) planes ( 111), ( 200), (220), and (311), as marked by the orange arrows in the figure [29].Furthermore, the diffraction peaks at 23.3°, 25.7°, 27.4°, and 33.7° are assigned to the MoO 3 planes (101), ( 400), (210), and (111) (JCPDS card No. 89-7112), confirming the presence of MoO 3 and aligning with HRTEM characterization results [31,34].Figure 3b presents an enlarged view of the (210) diffraction peak of MoO 3 , illustrating that as KBH 4 soaking time is extended, the diffraction peak shifts noticeably towards smaller angles.According to the Bragg equation, this shift is likely associated with lattice distortions due to high concentrations of oxygen vacancies [35].Noteworthily, the relative intensity of the diffraction peaks corresponding to MoO 3 gradually decreases, while the intensity of the characteristic diffraction peaks corresponding to the SiC nanowires does not show significant changes across the samples.It can be reasonable speculated that this phenomenon may be related to the following two reasons: first, the partial etching of MoO 3 when immersed in KBH 4 leads to a reduction in coating thickness and a decrease in the relative content of MoO 3 in the nanocomposite.Secondly, the introduction of oxygen vacancies into the MoO 3 lattice induces changes in the intrinsic crystal structure, Fig. 3 a XRD patterns of SiC@MoO 3 nanocomposite and SiC@MO-t samples.b Partial enlarged detail of (210) diffraction peak.c Raman patterns of SiC@MoO 3 nanocomposite and SiC@MO-t samples.XPS spectra of d O 1s and e Mo 3d.Ratio of the f Mo 5+ /Mo 6+ and g O defect content in different samples.h EPR spectra of SiC@MoO 3 nanocomposite and SiC@MO-t samples causing lattice distortion.In regions with a high concentration of oxygen vacancy defects, the disorderly arrangement of atoms is triggered, resulting in decreased crystallinity and weakened diffraction peaks.Figure 3c depicts the Raman spectra of the SiC@MoO 3 nanocomposite and SiC@MO-t samples within the 600-1200 cm −1 range.The characteristic peak near 850 cm −1 corresponds to the TO mode of the SiC nanowire core, while the peak near 1000 cm −1 is potentially associated with the stretching vibrations of [MoO 6 ] octahedral units comprising MoO 3 [36,37].Compared to the SiC@ MoO 3 nanocomposite, soaking in the reducing agent KBH 4 causes a shift of the Raman absorption peak associated with symmetric stretching vibrations towards lower wave-numbers.The longer the soaking time, the more pronounced the red-shift, indicating that the introduction of oxygen vacancies leads to a reduction in Mo 6+ cations.
XPS spectroscopy was employed to study the chemical bonding information and surface valence state changes of the SiC@MoO 3 nanocomposites and SiC@MO-t samples, with their corresponding total spectra are depicted in Fig. S3.Given the high stability of the SiC nanowires used, no significant changes were observed before and after soaking treatment.Therefore, only the O 1s and Mo 3d spectra were analyzed in detail, with the results displayed in Fig. 3d, e, respectively.The O 1s spectra derived from the SiC@MoO 3 nanocomposite and SiC@MO-t samples typically exhibit two distinct fitted peaks, corresponding to lattice oxygen (Mo-O) at 530.5 eV and defect oxygen (oxygen vacancies) at 532.2 eV.Importantly, the defect oxygen characteristic peak shifts towards higher binding energies as the duration of the reducing agent treatment is extended, indicating a rise in the concentration of oxygen vacancies in the products [38,39].In Fig. 3d, only Mo 6+ signals are observed in the SiC@MoO 3 nanocomposite, with fitted peaks at 233.2 and 236.3 eV corresponding to Mo 6+ 3d 5/2 and 3d 3/2 , respectively [40,41].In the produced SiC@MO-t samples, the characteristic peaks of Mo 6+ 3d 5/2 and 3d 3/2 shift towards lower binding energies, and characteristic signals of Mo 5+ 3d 5/2 , and 3d 3/2 emerge near 231.8 and 234.7 eV, potentially linked to the presence of oxygen vacancies in the lattice.To further explore the concentration of oxygen vacancies and the valence states of bonded atoms in different samples, the integral areas of various characteristic peaks were calculated, yielding the ratio of Mo 5+ /Mo 6+ to the content of defect oxygen, as depicted in Fig. 3f.Clearly, as the soaking time increases, the content of Mo 5+ and the concentration of oxygen vacancies increase.The ratio for SiC@MoO 3 nanocomposite materials is very small (almost negligible).When the immersion time is short (2 and 4 min), the ratio of the integrated area of the Mo 5+ /Mo 6+ characteristic peaks slightly increases in the SiC@MO-t2 and SiC@MO-t4 samples compared to the initial sample.However, with longer immersion times (6 and 8 min), the ratio of the integrated area of the Mo 5+ /Mo 6+ characteristic peaks for the SiC@ MO-t6 and SiC@MO-t8 samples rises sharply, reaching values several times higher than those of the SiC@MoO 3 nanocomposite materials.Figure 3g is the integrated area ratio of the O defect peak relative to the combined area of the O defect and Mo-O peaks, exhibiting the same trend as in Fig. 3f, with the SiC@MO-t8 sample having the highest ratio.To verify and quantify the concentration of oxygen vacancies in SiC@MO-t samples, EPR testing was employed as a reference method.As depicted in Fig. 3h, the g-factor for all samples remains consistently at 2.003, largely because the oxygen vacancies in all samples capture electrons [42,43].The figure clearly illustrates that the EPR signal of SiC@MoO 3 nanocomposite is significantly weak, suggesting a minimal presence of oxygen vacancies in the sample.With prolonged exposure to the reducing agent KBH 4 , the EPR signal of the SiC@MO-t samples increase in intensity, reflecting an upward trend in oxygen vacancy concentration, which corroborates the findings from earlier XRD and XPS analyses.

Microwave Absorption Performance of Oxygen
Vacancy-Rich SiC@MoO 3 Nanocomposites The effectiveness of electromagnetic wave absorbing materials can be assessed by examining the relationship among thickness, electromagnetic wave frequency, and reflection loss (RL) values.The formula employed is as follows [44]: (1) where Z in denotes the input impedance of the absorber, while Z 0 denotes the impedance of free space.The relative complex permittivity and relative complex permeability are denoted by ε r and μ r , respectively.Here, c denotes the speed of light, f denotes the frequency of the incident electromagnetic wave, and d denotes the thickness of the absorber.Figures S4 and S5 depict the relationship between the most pronounced minimum reflection loss (RL min ) and (2) effective absorption bandwidth (EAB) across varying matching thicknesses in SiC@MoO 3 nanocomposite and SiC@ MO-t samples, as functions of frequency.As depicted in the figures, each sample exhibits a significant frequency dispersion effect.Specifically, as the matching thickness increases, the RL min values progressively transition from the high-frequency to the mid-low frequency range.6 GHz) and features a preferred EAB of 1.04 GHz, with corresponding matching thicknesses of 5.43 and 5.32 mm, respectively.As depicted in Fig. 4b, the SiC@MO-t2 sample achieves an optimal RL min value of − 39.92 dB at a matching thickness of 2.41 mm and exhibits an optimal EAB value of 4.32 GHz at 2.07 mm matching thickness.At a matching thickness of 1.27 mm, the SiC@MO-t4 sample (Fig. 4c) exhibits the strongest RL min value of -50.49dB, with its optimal EAB value of 4.64 GHz occurring at a matching thickness of 1.42 mm.As shown in Fig. 4d, the SiC@MO-t6 sample displays the strongest RL min value of -49.83 dB at a matching thickness of 2.68 mm.At a matching thickness of 2.81 mm, its EAB value achieves an impressive 8.72 GHz (9.28-18 GHz), encompassing the entire Ku band.For the SiC@MO-t8 sample (Fig. 4e), the optimal RL min value is -35.96dB at a matching thickness of 5.41 mm, whereas the optimal EAB value is 2.48 GHz at a matching thickness of 2.58 mm.Previous reported work pointed out that the introduction of oxygen vacancies plays a critical role in modulating the frequency range over which electromagnetic wave attenuation occurs [45].Compared to the SiC@MoO 3 nanocomposite, the SiC@MO-t samples with increased oxygen vacancies exhibit a shift in their RL min values and preferred EAB from lower frequency (C-band) to the midrange (X band) or higher frequency (Ku-band).This shift is accompanied by reductions in their required matching thicknesses.These observations underscore the significance of oxygen vacancies in enhancing the electromagnetic wave absorbing performance of such materials.Obviously, the RL min and EAB values gradually shift from high frequency to low frequency, which is in accordance with the 1/4 wavelength theory (as exhibited in Fig. S6).To further explore the absorption characteristics of the absorber, the relationship between the matching frequency (ƒ m ) and the matching thickness (t m ) was analyzed by the following equation.
where t m is the matching thickness of the absorber, λ is the wavelength of the incident electromagnetic wave, C represents the speed of light in vacuum, and ƒ m is the corresponding matching frequency.It can be clearly seen that the perfect matching relationship between the thickness of the prepared series of samples and the propagation wavelength (λ/4, 3λ/4) is demonstrated, indicating that the quarter-wavelength theory effectively enables multi-band absorption performance.In comparison, SiC@MO-t4 and SiC@MO-t6 achieve optimal multi-band and multi-thickness effects, corresponding to excellent impedance matching capabilities of the material.They exhibit a wide range of exceptional matching performance in various bands and thicknesses.Figure 4f depicts the trends in RL min and EAB values at the optimal matching thicknesses for various samples.Interestingly, it reveals that the enhancement in electromagnetic wave absorption performance due to oxygen vacancies does not exhibit a monotonically increasing trend.Instead, as the concentration of oxygen vacancies increases, the optimal values of RL min and EAB values initially increase and subsequently decrease.To provide a more comprehensive and intuitive depiction of the electromagnetic wave absorption performance of each SiC@ MO-t sample, bar charts illustrating the changes in RL min and EAB values across various matching thicknesses were developed, as shown in Figs.4g and S7.As indicated, both the SiC@MO-t4 and SiC@MO-t6 samples, possessing a moderate concentration of oxygen vacancies, demonstrate the highest RL min and broadest EAB across multiple thicknesses, highlighting their superior absorption capabilities.Analysis of the XRD spectra and HRTEM images presented in Figs. 2 and 3 reveal that prolonged exposure to the reducing agent could result in an excessively high concentration of oxygen vacancies.This excess may disrupt the original MoO 3 structure, consequently leading to suboptimal electromagnetic wave absorption performance in the SiC@MO-t8 sample compared to the SiC@MO-t6 sample.As previously discussed, while a small number of oxygen vacancies function as dipoles in an alternating electromagnetic field and exert limited effects, an excess of oxygen vacancies not only increases the concentration in local areas of the MoO 3 shell but also significantly compromises its structural integrity.This results in an overall decrease in oxygen vacancy content within the shell.From both perspectives, there are limited improvements in the electromagnetic wave absorption performance of SiC@MoO 3 nanocomposite.In contrast, both the SiC@MO-t4 and SiC@MO-t6 samples, which contain a moderate level of oxygen vacancies, strike an optimal balance between concentration and content, thereby significantly enhancing the electromagnetic wave absorption performance of the initial product.Figure 4i and Tab.S1 present a comparative analysis of the electromagnetic wave absorption performance of the SiC@MO-t4 and SiC@ MO-t6 samples with recent reports on absorbers involving Mo and other oxides [46][47][48][49][50][51][52][53].The results indicate that the synthesized SiC@MO-t4 and SiC@MO-t6 samples exhibit a wider EAB and more impressive RL min values.These characteristics align with the ideal properties of absorptive materials, namely thinness, broad bandwidth, lightweight, and high efficiency.
The complex permittivity (εr = ε′−jε′′) and complex permeability (μr = μ′−jμ′′) are crucial parameters in evaluating the performance of electromagnetic wave absorbing materials.The real parts (ε′ and μ′) and the imaginary parts (ε′′ and μ′′) respectively denote the storage and attenuation capacities of these materials for the electric and magnetic energy of incident electromagnetic waves [54].These values are derived using the following formulas: herein, ε ∞ represents the optical permittivity, ε s indicates the static permittivity, τ signifies the relaxation time, and f denotes the frequency.Considering that both SiC nanowires and the MoO 3 shell are non-magnetic materials, their real and imaginary parts of permeability values exhibit minor fluctuations around 1 and 0, respectively [26].The nanocomposite demonstrates negligible magnetic loss to incident electromagnetic waves (Fig. S8), which can be reasonably disregarded in this analysis.This research focuses on the impact of dielectric loss on the attenuation of electromagnetic waves by these materials.Figure 5a, b depicts the frequency-dependent variations in ε′ and ε′′ values for the SiC@MoO 3 nanocomposite and various SiC@MO-t samples, respectively.At low frequencies, the dielectric constant increases with increasing frequency because molecular polarization requires a certain amount of time.Conversely, at high frequencies, the dielectric constant decreases with increasing frequency because the relaxation time for molecular polarization is too short to complete polarization [55][56][57].The ε′ and ε′′ values of the pure SiC@MoO 3 nanocomposite remain stable at approximately 4 and 0.13, respectively.This stability may account for the lack of frequency-dependent variations in the RL min values observed in Figs.3a and S4.The remarkably low ε′′ value of the sample results in insubstantial dielectric loss to incident electromagnetic waves, posing challenges to achieving satisfactory wave absorption performance.Furthermore, a difference in values exceeding 20-fold indicates significant alterations to the impedance matching characteristics [58].With the introduction of oxygen vacancies, the ε′ value of the SiC@MO-t samples rises markedly, indicating an enhanced capacity for electric field energy storage.Within the 2-9 GHz frequency range, the ε′ values of the samples remain stable, displaying smooth curves, with the SiC@MO-t4 sample exhibiting significantly higher values than the others.Meanwhile, the SiC@MO-t6 sample exhibits values comparable to those of SiC@MO-t8 sample in this frequency region, suggesting their indistinguishable electric field storage capability.In the frequency range of 9-12 GHz, most of the SiC@MO-t samples (SiC@MO-t2 ~ SiC@MO-t6) exhibit a gradually decrease in ε′ values.For the SiC@MO-t8 sample, the declining trend commences at approximately 12-14 GHz.Additionally, throughout the mid to high frequency range, all SiC@MO-t samples display multiple distinct resonance peaks, potentially linked to polarization effects induced by oxygen vacancies [2].As illustrated in Fig. 5b, the ε′′ values of the SiC@MO-t samples initially increase and subsequently decrease.With an increase in the concentration of oxygen vacancies, the ε′′ values of the SiC@MO-t samples exhibit an overall trend of initially increasing and then decreasing.Notably, the SiC@MO-t4 sample demonstrates the highest ε′′ values compared to the other samples across the entire measured frequency range, indicating its superior attenuation capacity for the electric field energy of incident electromagnetic waves, with a peak value near 10 GHz, approximately 12.9 [59].Throughout the entirety of the testing frequency range, the ε′′ values of the SiC@MO-t6 sample consistently range between 2 and 4, indicating its stable attenuation capacity for the electric field energy of incident electromagnetic waves.Additionally, the maximal ε′′ values of each SiC@MO-t sample are observed within either the X or Ku band, accompanied by multiple distinct resonance peaks.In Fig. 5a, the ε′ values of SiC@MO-t2 and SiC@MO-t8 are either slightly higher than or equivalent to those of the SiC@MO-t6 sample across the entire range of tested frequencies.However, in Fig. 5b, the values of the former two are consistently lower than those of the latter throughout the entire tested frequency range.This suggests Page 12 of 21 https://doi.org/10.1007/s40820-024-01478-2© The authors a more pronounced disparity between their capabilities in storing and attenuating electric field energy, which may significantly influence the impedance matching characteristics of the materials and potentially have a detrimental effect on electromagnetic wave attenuation.Figure 5c presents the frequency-dependent curves of the loss tangent values (tanδ ε = ε′′/ε′) for various products.Within the 2-10 GHz frequency range, a uniform trend is observed across all samples.The maximum tanδ ε values for the SiC@MO-t samples with oxygen vacancies are observed within the 10-13 GHz i Attenuation constant of SiC@MoO 3 nanocomposites and SiC@MO-t samples.j-m Delta value of SiC@MO-t samples frequency range, exhibiting a variation tendency similar to that of various samples exhibited in Fig. 5b.It is noteworthy that, in contrast to the results in Fig. 5b, the SiC@MO-t4 sample presents two distinct peaks in tanδ ε values, and the disparity in tanδ ε values between the SiC@MO-t4 and SiC@ MO-t6 samples is notably reduced.These findings indicate that, in addition to ε′′, other parameters also significantly impact the electromagnetic wave attenuation characteristics.
Generally, the dielectric loss in wave-absorbing materials can be primarily attributed to conductive and polarization loss [60].To elucidate the attenuation characteristics of incident electromagnetic waves on varied SiC@MO-t samples, resulting from oxygen vacancies, the values of conductive and polarization loss as functions of frequency were calculated using the prescribed formula.In this context, ε p′′ denotes polarization loss, ε c′′ denotes conductive loss, σ represents the conductivity, and ε 0 (ε 0 = 8.854 × 10 −12 F m −1 ) represents the permittivity of free space [61].
The relationships between the conductive and polarization loss of the various samples as functions of frequency are depicted in Fig. S9, respectively.Notably, the ε p′′ and ε c′′ values of the SiC@MoO 3 nanocomposite remain stable with increasing frequency, maintaining low levels, and their combined impact on dielectric loss is negligible.This observation aligns with the trends depicted in Fig. 5a, b.Furthermore, as the frequency increases, the ε c′′ values of the various SiC@MO-t samples gradually decrease, indicating a diminishing contribution of conductive loss to the overall dielectric loss.The ε c′′ values of the SiC@MO-t4 sample consistently exceed those of other samples across all tested frequency ranges, indicating its pronounced conductive loss capability.The ε c" values of SiC@MO-t2, SiC@MO-t6, and SiC@MO-t8 closely align, and given the notable differences in their electromagnetic wave absorption capabilities, it can be inferred that polarization loss plays a predominant role in mitigating the effects of incident electromagnetic waves on these samples.Figure S9b illustrates the polarization loss curves of each SiC@MO-t sample plotted against frequency, showing that the ε p" values follow the trends depicted in the Fig. S9a.Notably, the polarization loss values in the SiC@ MO-t2, SiC@MO-t6, and SiC@MO-t8 samples exceed their respective conduction loss values by a factor of one, confirming that polarization loss associated with oxygen vacancie predominates as the source of dielectric loss in these materials.Despite minimal difference in conduction loss values, the polarization loss values of the SiC@MO-t2 and SiC@ MO-t8 samples are notably higher than that of the SiC@ MO-t6 sample.This contrasts with the preceding results on reflection loss, implying that impedance matching associated with oxygen vacancies may exert a more significant influence on the enhancement of absorption performance than on dielectric loss performance.For the SiC@MO-t4 sample, in the S-band, its conduction loss values surpass its polarization loss values, whereas in the remaining measured bands, the contribution of polarization loss to dielectric loss is predominant over conduction loss.The conductivity (Fig. S10) of the SiC@MoO 3 nanocomposite is notably lower than that of the SiC@MO-t samples containing oxygen vacancies, and its conductivity is correlated with the sequence of curves in Fig. 5b.This effect is likely attributed to oxygen vacancies that introduce defect levels into the band-gap of the MoO 3 semiconductor, enhancing the migration and hopping capacity of surface electrons and thus resulting in significantly improved conductive loss characteristics [62].
To examine the polarization loss of the samples, the relaxation behavior of the various products was analyzed using Debye relaxation theory.The typical relationship between ε′ and ε′′ can be described by the following equation [45]: Figure S11 exhibits the Cole-Cole curves of SiC@MoO 3 nanocomposites.It can be observed that a single semicircle can be marked, suggesting that limited Debye relaxation phenomena occur, which cannot effectively dissipate the energy of the incident electromagnetic wave.The Cole-Cole plots of SiC@MoO 3 nanocomposites and SiC@MO-t samples are depicted in Fig. 5d-g, respectively.It can be found that the introduction of oxygen vacancies in the SiC@MO-t sample results in additional semicircles, indicating a more pronounced Debye relaxation phenomenon relative to the SiC@ MoO 3 nanocomposites.Similar to the results presented in Fig. 5b, the SiC@MO-t4 and SiC@MO-t6 samples with optimal oxygen concentration, exhibit the most semicircles, indicating the strongest relaxation behavior among them and corresponding to better electromagnetic wave loss properties [63].Furthermore, the long-tail phenomenon at the end of the Cole-Cole curve indicates the conduction loss of various samples, representing the conductivity loss contributing to electromagnetic wave attenuation of the absorber [64].Analysis of the graph reveals that the calculated slope values of the samples are correlated with the relative magnitude of their conduction loss.Additionally, the different Cole-Cole curve tail slope values were calculated, as indicated by the blue line in Fig. 5h, revealing that SiC@MO-t4 sample exhibits strong conduction loss.It is well known that the excellent absorption performance of absorptive materials depends on two important parameters: impedance matching, which reflects the extent of electromagnetic waves penetrating the absorbing material, and the attenuation coefficient, which reflects the attenuation capacity of the absorber for incident electromagnetic waves.These two parameters may be calculated using the following formulas [65]: Figure 5i depicts the attenuation coefficient variations of the SiC@MoO 3 nanocomposite and SiC@MO-t samples.Across the entire frequency range, the α values for each SiC@MO-t sample exceed those of SiC@MoO 3 nanocomposite, with the relative positions of the curves correspond to those in Fig. 5b, which suggests a stronger attenuation capacity for the incident electromagnetic waves.These findings provide further evidence that oxygen vacancies promote electromagnetic wave loss, consistent with the observations from the polarization loss curve shown in Fig. S9.The maximum value for the SiC@MoO 3 nanocomposite is observed at approximately 6 GHz, reaching nearly 100.In other examined frequency ranges, the values are relatively small and exhibit negligible variation, indicating a poor attenuation capability against incident electromagnetic waves.In the S-band, the attenuation coefficient values for each SiC@MO-t sample appear to be comparatively uniform.In the C and X bands, the attenuation coefficient values for each SiC@MO-t sample demonstrate a gradual increase with respect to frequency.In the Ku frequency band, the attenuation coefficient values for each SiC@MO-t sample tend to fluctuate in proximity to their maximum values.The delta function (|Δ|) was computed to evaluate the impedance matching of SiC@ MoO 3 nanocomposite with various SiC@MO-t samples with the following equation [66]: The constants K and M can be determined from the relationship between complex permittivity and complex permeability, as indicated in the following equation: Optimal impedance matching in the absorbers is achieved when the absolute value of |Δ| approaches zero.Effective impedance matching for electromagnetic wave absorbing materials occurs when |Δ| falls below 0.4.Figure 5j-m depicts the |Δ| values for a range of samples under optimal conditions.Notably, the SiC@MO-t4 and SiC@MO-t6 samples exhibit a broader range where |Δ| is less than 0.4, suggesting enhanced impedance matching that enables incident electromagnetic waves to penetrate the material more effectively.This indicates that while excellent attenuation capacity is crucial for superior electromagnetic loss performance, combing it with excellent impedance matching is equally fundamental [67].Based on the aforementioned evidence, one can conclude that oxygen vacancies have a significant impact on both the attenuation coefficient and the impedance matching characteristics of SiC@MoO 3 nanocomposite.

Electromagnetic Wave Attenuation Mechanism of Oxygen Vacancy-Rich SiC@MoO 3 Nanocomposites
To comprehensively understand the impact of oxygen vacancies on electromagnetic wave absorption, the electronic distribution around these vacancies in various samples was investigated using density functional theory (DFT) calculations.Since identical SiC nanowires acted as carriers or substrate, simplifying the model, the alterations in the electronic structure of MoO 3 across different samples were investigated.Figure 6a-d depicts atomic structure models and band structures of intrinsic MoO 3 and MoO 3 with oxygen vacancies.It is well known that in semiconductor materials, oxygen vacancies (V O ) can reduce the hybridization energy level, thereby decreasing the band gap [68].Under the influence of oxygen vacancies, neighboring electrons around the low-coordination Mo atoms enter the conduction band, thereby increasing the conductivity.Consequently, the band gap of MoO 3 in the SiC@MO-t sample decreases from the direct band gap of intrinsic MoO 3 with the value of 2.27 to 0 eV.This narrowing facilitates electron transfer and hopping, enhancing the conductivity loss effect of electromagnetic wave [69].Therefore, compared to SiC@ MoO 3 nanocomposites, the conductivity loss of SiC@MO-t samples is significantly enhanced.Figure 6c, d depicts the density of states (DOS) for bare MoO 3 and MoO 3 with oxygen vacancies, respectively.For samples containing oxygen vacancies, it can be observed that the valence band maximum is mainly composed of O-2p orbitals.In the energy region higher than the O-2p orbitals, some new Mo-4d and O-2p orbitals appear, indicating that oxygen vacancies cause additional Mo-4d and O-2p electronic orbitals, resulting in a narrowing the band gap of system.From Fig. 6e, f, it can be seen that for intrinsic MoO 3 , the charge density distribution shows no significant differences across the entire model.On the other hand, Fig. 6g, h displays the charge difference for MoO 3 with oxygen vacancies, clearly indicating charge transfer at the positions of missing oxygen atoms.This implies that the absence of lattice oxygen disrupts the symmetric charge distribution, leading to the induction of electric dipoles and the formation of positive charge centers, which act as enhanced dipole polarization in alternating electromagnetic fields, contributing to the attenuation of incident electromagnetic waves [20].
Based on the above analysis, the SiC@MO-t sample can be regarded as a nanocomposite consisting of SiC@MoO 3 coupled with oxygen vacancy defects.The impact of SiC@ MO-t on incident electromagnetic waves results from the combined dielectric loss performances.For the SiC@MoO 3 nanocomposite, the mechanism diagram illustrating its role in attenuating electromagnetic waves is shown in Fig. 6i.The main mechanism can be summarized as follows: (1) Structurally, the densely packed SiC@MoO 3 one-dimensional nanomaterials form an interwoven network.The disordered pore structure, surrounded by adjacent nanowires, functions as a transmission channel for electromagnetic waves to penetrate into the interior of the network.Simultaneously, it enhances multiple reflections and scattering of electromagnetic waves, effectively improving the dielectric loss capacity of the product [70].(2) While both β-SiC and α-MoO 3 are wide band-gap semiconductor materials, the band-gap of latter is approximately 2.72 eV, smaller than that of the former, which has a value of 3.13 eV.Compared to single SiC nanowires, the synthesized SiC@MoO 3 one-dimensional nanomaterials exhibit a narrower band-gap [71,72].This reduction diminishes the energy barriers for electron migration and hopping, potentially enhancing the electrical loss characteristics of the sample to a certain extent.(3) Due to the significant difference in electron transfer efficiency between SiC and MoO 3 , charge carriers may become trapped at the hetero-junction interface during migration.This leads to the accumulation and non-uniform distribution of charges around the interface region, which in turn generates abundant interface polarization losses [73].(4) Within SiC nanowire structures, stacking faults, along with atoms in the perfect crystal lattice, may serve as polarization centers.This induces the redistribution of positive and negative charges, creating imbalances in dipole moments and subsequent dipole polarization formation [25,70].This mechanism further contributes to enhanced dielectric loss properties of the product.Oxygen vacancy defects in the SiC@MoO 3 sample play a crucial role in enhancing its absorption performance compared to the SiC@MoO 3 nanocomposite.Firstly, oxygen vacancies modify the intrinsic band structure of MoO 3 semiconductors by introducing defect energy levels, thereby reducing the energy barrier for electron transitions [74].This enhancement in electron transport results in stronger electrical loss capabilities relative to SiC@MoO 3 nanocomposite.Furthermore, the introduction of oxygen vacancies in the MoO 3 lattice induces lattice distortion and serves as polarization centers.These vacancies capture and release charge carriers, altering the charge distribution around them and generating strong defect-induced dipole moments [12].This emphasizes the role of polarization loss in attenuating electromagnetic waves.Moreover, the presence of oxygen vacancies facilitates the tuning of impedance matching between the SiC@MO-t sample and its surrounding environment, enabling enhanced penetration of electromagnetic waves that impinge on the surface of the material into the absorber.This optimizes the dissipation of electromagnetic wave energy.
To demonstrate the effectiveness of SiC@MoO 3 nanocomposite and SiC@MO-t samples, which are rich in oxygen vacancies, in broadband and wide-angle electromagnetic wave absorption, this study also aims to verify the stealth performance of the absorbers in practical applications.Under the detection frequency of 14.96 GHz, 3D far-field simulation maps of the samples were perfectly simulated (Fig. 7c-h).Figure 7a shows the establishment of a perfect PEC plate model.Figure 7b shows the PEC model and the maximum RCS value of SiC@MO-t sample as − 32.93 dB m 2 .Generally, the radar cross-section (RCS) value of a material represents the physical quantity of the F-reflected wave intensity detected by radar under aircraft coating conditions, and a smaller RCS value indicates better stealth performance of the material.The RCS of the aircraft is related to the unit solid angle reflection in the radar receiving direction [75].Additionally, the basic equation describing the radar and the radar intercept signal value, as well as the power density of target firing, are relevant.The equation for calculating the cross-section (RCS, σ) can be expressed as [76]: In the formula, E s and E i represent the intensity of scattered electric field and incident electric field respectively.H s and H i represent the intensity of scattering c magnetic field and incident magnetic field respectively.S s and S i represent the power densities of scattering and incident lengths.Accordingly, a 3D far-field simulation was conducted at a detection frequency of 14.96 GHz for five materials.Since kα is greater than 10, and the target size of the aircraft model is larger than the signal wavelength [77].In addition, the forward (Fig. 7j) and top-down (Fig. 7k) RCS (measuring the transverse electropolarized waves of the returned sensor) of the UAV at the optimal frequency (14.96GHz) show that the composite structure reflects a weaker electromagnetic wave signal than the basic PEC model.The results show that SiC@MO-t series absorbing materials rich in oxygen vacancies have excellent (15) electromagnetic wave absorption capability, and exhibit excellent attenuation strategy to effectively regulate the radio wave absorption under complex conditions.

Conclusion
In this study, SiC@MoO 3 nanocomposites with controllable vacancy concentration were synthesized by the in situ growth of MoO 3 on the surface of pre-obtained SiC nanowires, followed by reduction using KBH 4 .Characterization results indicate that the presence of oxygen vacancies directly enhances dipole polarization, thereby improving the effectiveness of electromagnetic loss.Analysis reveals that the optimized SiC@MO-t4 nanocomposite exhibits superior electromagnetic wave absorption performance, achieving an RL min value of -50.49dB at a matching thickness of 1.27 mm.The SiC@MO-t6 nanocomposite exhibits exceptional performance, with an impressive EAB of 8.72 GHz at a matching thickness of 2.81 mm, covering the entire Ku band.Remarkably, both nanocomposites outperform the SiC@MoO 3 sample in terms of electromagnetic loss efficiency.Furthermore, the exploitation of vacancy-induced direct dipole polarization presents a promising approach for enhancing electromagnetic wave absorption capabilities through defect engineering.This strategy holds potential applications in the development and enhanced performance of other absorbers.

Fig. 1 a
Fig. 1 a Schematic illustration of SiC@MoO 3 nanocomposite and SiC@MO-t samples synthesized at different KBH 4 soaking time.b-d SEM images of SiC@MoO 3 nanocomposite.e-i C, Si, Mo, and O element mappings of SiC@MoO 3 nanocomposite.j Content of different elements in SiC@MoO 3 nanocomposite.k-n Representative TEM and HRTEM images of SiC@MoO 3 nanocomposite Fig.4a-e RL values versus frequency of SiC@MoO 3 , SiC@MO-t2, SiC@MO-t4, SiC@MO-t6 and SiC@MO-t8 at different thickness.f Comparison of RL min and EAB value of different samples at optimal matching thickness themselves.g RL min comparison of different samples at various matching thickness.h Comparison of RL min and EAB values of the SiC@MO-t4 and SiC@MO-t6 samples with recent reports related absorbers

Fig. 5 a
Fig. 5 a Real part and b imaginary part of complex permittivity of different SiC@MO-t samples.c Dielectric loss tangent, d-g Cole-Cole curve of SiC@MO-t samples.h Trailing slop of Cole-Cole curve.i Attenuation constant of SiC@MoO 3 nanocomposites and SiC@MO-t samples.j-m Delta value of SiC@MO-t samples

Fig. 6
Fig. 6 Energy structure of a SiC@MoO 3 nanocomposite and b SiC@MO-t sample.Projected density of state of c SiC@MoO 3 nanocomposite and d SiC@MO-t sample.e Atomic structure and f charge distribution of SiC@MoO 3 nanocomposite.g Atomic structure and h differential charge density of SiC@MO-t sample.i Schematic diagram of electromagnetic wave attenuation mechanism